1. Field of the Invention
The present invention relates to a nonselective chemical composition useful in the chemical management of plant diseases. More specifically, the chemical composition of the present invention incorporates a class of naturally occurring alkaloids having antimicrobial properties.
2. Description of the State of the Art
A plant disease may be defined as any disturbance that prevents the normal development of a plant and reduces its economic or aesthetic value. Infectious plant diseases are caused by living agents (microorganisms), or pathogens, such as, nematodes, fungi, bacteria, mycoplasmas, viruses and viroids, which interfere with the normal function of some part of the plant, resulting in lower yields or reduced quality. Although there are at least 50,000 diseases of economic plants, and new diseases are discovered every year, it is difficult to accurately assess losses from disease. However, it is safe to say that annual losses in the United States average about 15% of the total agricultural production, or more than about $15 billion. Disease causes another $150 million in economic loss in terms of the replacement value and increased maintenance costs of ornamentals and turf in the United States. For the farmer, this leads to less income; for the consumer, reduced food supplies of lower quality and higher prices; and for the homeowner and turf manager, lower aesthetic value, less beauty and higher maintenance costs.
The idea of using chemicals to protect plants from microorganisms or "pests" goes back at least 2000 years. Homer, the Greek poet and author of The Illiad and The Odyssey, wrote about the "pest averting sulfur with its property of divine and purifying fumigation." Broadly speaking, chemicals that are used to kill or inhibit the development of microorganisms are referred to as pesticides. As biological knowledge grew in the eighteenth and nineteenth centuries and as pest problems became more severe as a result of more complex agriculture and the introduction of pests into new areas, the search for effective pest management methods intensified. The discovery in 1882 that spraying grapes with a Bordeaux mixture would reduce damage from downy mildew encouraged scientists to look for other chemicals that would kill pests. Certain chemicals are useful as pesticides because they are more toxic to plant pathogens than they are to plants. Some pesticides are nonselective, that is, toxic to many pathogens; and others are selective, affecting only a few or one kind of pathogen.
In the twentieth century plant pathology matured as a science with startling and rapid discoveries. Since the 1930s, many hundreds of compounds have been screened for chemotherapeutic activity. In most studies, made with the aim of discovering compounds of practical use in the field, activity was measured directly by application to the growing plant; however, this proved to be a slow and laborious screening process. In response, scientists developed more rapid and economic techniques, such as, floating inoculated leaf discs on solutions of the test compounds or using excised petioles, for detecting the movement of antimicrobial chemicals within the plant body.
To date the vast majority of compounds utilized as pesticides have been synthetic organic and inorganic compounds. Because of the growing concerns regarding pesticide damage to the environment scientists have shifted their focus to examining constituents from higher plants for chemotherapeutic activity. Many of these constituents have been implicated in the natural resistance which is shown by plants towards most pathogens. Greathouse, G. A., demonstrated in vitro that quartenary benzophenanthridine alkaloids, the sources of which include five plant families: Papaveraceae, Fumariaceae, Rutaceae, Capifoliaceace, and Meliaceae, influenced the growth of several fingal species, and root rots known as Phytomatoryrichum omnivorum. See, Greathouse, G. A., et al., "The Chemistry of Resistance of Plants to Phymatotrichium Root Rot v. Influence of Alkaloids on Growth of Fungi," Phytopathology 30:475-485 (1940); and Greathouse, G. A., "Alkaloids from Sanguinaria canadensis and Their Influence on Growth of Phymatotrichium omnivorum," Plant Physiology, 14:377-380 (1939), respectively. In addition to these in vitro studies, plant extracts containing benzophenanthridine alkaloids have been utilized in a number of pharmaceutical compositions for human medical and veterinary treatment applications including ringworm, dysentery, expectorants, scours, antiplaque and anti-gingivitis in oral health care, and anti-inflammation.
The benzophenanthridine alkaloids have been shown to be active in vitro against several fungal and bacterial strains. The minimum concentrations of benzophenanthridine alkaloids inducing complete inhibition of visible growth on Mueller-Hinton media are given in Table 1 for a variety of pathogens.
TABLE 1 IN VITRO ANTIMICROBIAL ACTIVITY OF BENZOPHENANTHRIDINE ALKALOIDS MINIMUM ANTIMICROBIAL CONCENTRATION OF BENZOPHENANTHRIDINE ORGANISM ALKALOID ug/ml Actinomycetes 6.25-25 Penicillium 50-100 Aspergillus 25-100 Botrytis cinera 5-50 Rhizoctonia solani 5-25 Verticillium dahliae 15-50 Fusarium 5-25 Erwinia 20-40 Alternaria 20-50 Dendryphion 20-50 Plasmopara 6.25-25 Phytomatotrichum omnivorum 2.5-10 Pseudomonas 175-512 Klebsiella 17-175 Bacillus 2-12 Sclerotium 16-32 Verticillium albo-atrum 16-32 Fusarium vasinfectrum 64-256 Pythium 40-100 Septoria 5-25 Sphaerotheca pannosa 5-25 Phytophthera 16-32
Years following Greathouse's discoveries a technical paper by Spencer, et al., was published demonstrating that the compound wyerone, isolated from broad bean tissue also had in vitro antifungal activity. See, Spencer, D. M., et al., "An Antifungal Substance from the Tissue of Vicia faba," Nature, Lond., 179:651 (1957). Since then, naturally-occurring L-amino acids, other than methionine, which have shown little activity and D-isomers which have usually proved more effective have also been studied. Certain analogues of natural amino acids show useful activity; some of these, such as, L-threo-.beta.-phenylserine, apparently operate through an effect on the host, while others, such as, canavanine, ethionine and fluorophenylanine, are fungitoxic antimetabolites which exert a direct action against the pathogen.
Unfortunately, the in vitro studies performed by Greathouse et al., Spencer et al., and others have limited use, since there is rarely a high correlation between in vitro microbial toxicity and therapeutic activity. An additional limitation and complication in the identification of compounds that may be used as pesticides is that chemicals are not usually used in their pure forms, but rather are mixed with inert substances to form pesticide formulations. Pesticide formulations must be prepared so the user can apply it in a safe, convenient, and effective manner.
Many factors affect the ability to place the pesticide on the target in the manner and amount for the most effective results, with the least undesirable side effects, and at the lowest possible cost. While the selection and use of equipment is of utmost importance, successful application is impossible without proper consideration to compatibility and to formulations which (1) are residual and active for a period of time, (2) are adhesive to the plant surface, (3) have good spreading properties, (4) are stable against photodeactivation, (5) have a low phytotoxicity, and (6) are capable of penetrating the plant tissues.
There is still a need, therefore, for a safe, convenient, effective, and nonselective chemical composition for the treatment of plant disease wherein the active ingredients are naturally occurring organic compounds.